It's simply because the cinema hasn't had its times confirmed yet. This is probably becuase they're waiting to know how many prints they're getting, or how many digital prints etc.
It could also be that they are having premier seats installed after Potter - this causes all the seating to be renumbered rendering bookings useless. My Odeon is having new seats during June, so we can't open booking for Potter or Shrek yet. There's more sites missing from the Shrek booking list, too.